Steinberg UR-RT4 Loopback function is OFF on iPhone 15 Pro Max

Hi there. I am a newbie here in this group/ forum.
Can someone help me figure out on how to turn on the loopback function on my iPhone 15 through DspMixFx App?
I am fully aware that ''cc mode is turned off" when using a iOS device like an iPad or an iPhone.
Is there any way, I can turn on its loopback function?
I am using the Steinberg UR-RT4 Audio Interface with my iPhone 15 Pro max while doing karaoke live on Facebook and tiktok.
I could hear myself clearly while singing and its background music, but my viewers only hear my vocals but not the background music or sounds of the song I sing.
My thought is I will purchase a new mixer with a built-in loopback button like the Yamaha AG06 and connect this with my Steinberg UR-RT4. Not so sure if this is applicable and the best thing to do to achieve the loopback function. I need help. Thank you.